Fin attachment structure and fin

ABSTRACT

To provide a fin attachment structure capable of attaching a fin detachably and easily on a board without using any tools, and particularly, to a fin attachment structure capable of reducing the damage to human body as much as possible when the fin comes into contact with the human body during surfing etc. and having an extremely high safety, and a fin. 
     The above problem can be solved by a fin attachment structure and a fin, in which a groove part is provided on the bottom surface of the surfboard and a hollow part  10  is provided on the inner surface thereof, the hollow parts being further provided at the opposing positions on both side surfaces of the groove part, and an elastic member is provided at the deep side within the hollow part and a bail part at the opening side for biasing in the direction in which it projects to outside, wherein by the insertion of the fin attachment base portion into the groove part, the fin is biased and sandwiched and held by the ball part.

TECHNICAL FIELD

The present invention relates to a fin attachment structure of surfboard and, more specifically, to an attachment structure with which a fin can be detachably attached to a surfboard, wherein the fin readily comes off in an emergency such as when the fin hits against human body, and a fin used therein.

BACKGROUND ART

Conventionally, there have been roughly two types of fin attachment structure of surfboard: a fixed fin type; and a detachable fin type.

The detachable fin type is used more frequently compared to the fixed fin type and there are various attachment structures because it allows the attachment of a fin in an arbitrary shape according to a user's preference and the fin can be detached and carried.

That is, a fin box etc. is arranged in advance on the bottom surface of a surfboard by embedding it and then the fin is mounted in the fin box so that it can be attached detachably.

In this case, structures that allow a fin to be detachable when the fin is broken or carried include those disclosed in Japanese Unexamined Patent Publication (Kokai) No. 16-255575 document 1), Translated Japanese Publication of Patent Application No. 2001-520961 (patent document 2), Translated Japanese Publication of Patent Application No. 2002-530242 (patent document 3), and Japanese Unexamined Patent Publication (Kokai) No. 2003-306195 (patent document 4).

These have a structure in which screws, bolts, etc., are used to fasten a fin when fixing the fin to the fin box.

In addition to the above, structures in which grooves having various structures are provided in the fin box and the fin is fastened fitting the fin into the groove include those disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2005-74026 (patent document 5), Japanese Unexamined Patent Publication (Kokai) No. 2005-112206 (patent document 6), and Japanese Unexamined Patent Publication (Kokai) No. 2006-280839 (patent document 7).

Further, as a structure in which the fin is fastened using a C-shaped clamp, there is one disclosed in Translated Japanese Publication of Patent Application, No. 2003-528003 (patent document 8).

In addition to the above, structures in which the fin is fastened so that it can be attached detachably using a special locking member include those disclosed in Translated Japanese Publication of Patent Application No. 2005-508798 (patent document 9) and Translated Japanese Publication of Patent Application No. 2005-526664 (patent document 10).

Furthermore, as a structure in which both the fin box and the fin fitted therein have a fitting structure in a special shape and thus the fin is fastened so that it can be attached detachably, there is one disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2007-160969 (patent document 11).

Patent document 1: Japanese Unexamined Patent Publication No. H6-255575

Patent document 2: Translated Japanese Publication of Patent Application No. 2001-520961

Patent document 3: Translated Japanese Publication of Patent Application No. 2002-530242

Patent document 4: Japanese Unexamined Patent Publication (Kokai) No. 2003-306195

Patent document 5: Japanese Unexamined Patent Publication (Kokai) No. 2005-74026

Patent document 6: Japanese Unexamined Patent Publication (Kokai) No. 1005-112206

Patent document 7: Japanese Unexamined Patent Publication (Kokai) No. 2006-280839

Patent document 8: Translated Japanese Publication of Patent Application No. 2003-528003

Patent document 9: Translated Japanese Publication of Patent Application No. 2005-508798

Patent document 10: Translated Japanese Publication of Patent Application No. 2005-526664

Patent document 11: Japanese Unexamined Patent Publication (Kokai) No. 2007-160969

DISCLOSURE OF THE INVENTION

As described above, there exist various attachment structures as to surfboard fins.

The structure disclosed in patent document 1 adopts so-called screw fixing and in which a screw is screwed up, or a fin can be detached by unscrewing the screw.

Because of this, attachment and detachment of a fin require a special tool, such as a screwdriver.

Further, when the fin comes into contact with human when it is in use, the fin will remain fastened and human body may be damaged, that is, it is unlikely that the fin comes off the board to prevent the fin from damaging human in such an emergency.

Next, in the structures disclosed in patent document 2, patent document 3, and patent document 4, a fin is screwed up in the transverse direction and detachment of the fin similarly requires a special tool, such as a screwdriver, and even if the surfer falls during surfing and the fin of the surfboard comes into contact with human body, the fin will remain fastened thereto and there is a possibility that the fin may damage human body.

Further, the structures shown in patent document 5 and patent document 6 are those in which a fin is inserted along a groove and fastened to a predetermined position and the fin can be tightly attached, however, is hard to come off.

That is, the structure is not classified as the group of screw fixing but on the other hand, in which the fin is tightly inserted into the recess portion in order to prevent the fin from dislocating unnecessarily when in use and fixed in place in the groove.

Consequently, the fin is tightly inserted in order to prevent the fin from dislodging and falling, and therefore, it is unlikely that the fin readily comes off the board by the impact caused by a collision etc.

That is, the fin comes off only when it receives the impact in the particular, special direction and if moves, the distance of movement is such that the fin moves more or less for the impact in the direction in which the fin comes off and the fin stops after the movement, and therefore, the damage to human body by the fin when it comes into contact with the human body cannot be prevented.

Next, as to patent document 7, the fin is similarly inserted into the groove and moved along the groove to be attached.

In this case, it is necessary to attach/detach the fin in accordance with the insertion procedure and further, the fin is hard to come off when in use for an unexpected impact unless it receives the impact in one particularly specified direction.

Consequently, when the fin comes into contact with human body, there is a high possibility that the fin remains attached on the board and the human body is damaged.

Next, in the configuration shown in patent document 8, the C-shaped clamp is used to fasten the fin to the fin box as a locking device, that is, the fin is tightly fitted into to be attached to the clamp in the so-called fin box, and a large force is required to detach the fin and at the same time, it is difficult to detach the fin unless the fin is pulled out in one certain direction of the fin box.

Consequently, such an impact caused when the fin comes into contact with human using the surfboard or an impact in a certain direction will not cause the fin to come off and therefore the fin remains fastened thereto.

Further, there arises another need, such as that at least the accuracy at the time of attachment is required, that is, the slot must be matched or the fin must be inserted tightly into the clamp.

According to patent document 9, a cam is rotated by the worm gears and thus the fin is fixed, and therefore, the rotation of the worm gears is necessary to attach/detach the fin and at the same time, the fin is fastened to the board and unlikely to come off by the impact when in use.

Consequently, it is not possible to dislodge the fin in an emergency.

According to patent document 10 also, a tag pin is put into the pin slot and tightened by the club screw, and thus the fin is fastened tightly thereto, however, the fin is unlikely to come off and fall or to be dislodged in an emergency.

Further, as to patent document 11, the fin can be attached easily, however, its attaching is done by inserting the fin in the obliquely backward direction and if the screw is not used to fix it, there is a possibility that the fin comes off only when a force is applied in one certain direction in an emergency.

Consequently, when the fin comes into contact with human body in variety of unexpected directions, that is, when the fin comes into contact with human body when the surfboard is used, the fin remains in an almost fixed state, and therefore, the contact with human body cannot be dealt with.

As described above, a certain fin attachment structure is desired, which causes the fin to readily come off to prevent damage to human body, that is, which causes the fin to come off the surfboard in a safe manner when in use.

Further, it is originally required for the fin to be mounted tightly to the surfboard without unexpectedly coming off other than in an emergency, such as at the time of contact.

Furthermore, the attachment structure needs to be one in which the fin can be attached/detached readily for replacement and is fastened securely to the surfboard as shown in the prior art.

The invention of claim 1 according to the present invention is a fin attachment mechanism for attaching a fin detachably on the bottom surface of a surfboard, comprising a groove part into which a fin attachment base portion is inserted on the bottom surface of the surfboard and a hollow part in the inner surface of the groove part, the mechanism being made up of a fin attachment structure in which the hollow parts are provided in the opposing positions on both side surfaces of the groove part, respectively, and an elastic member is provided at the deep part in the hollow part and at the same time, a ball part is provided at the opening side, wherein the ball part is biased by the elastic member from the opening in the hollow part so that it projects in the outward direction, and by the insertion of the fin attachment base portion into the groove part, the ball parts in the hollow parts provided on both sides of the groove part are moved into the hollow parts against the biasing of the elastic member and thus the fin is biased by the ball part and is sandwiched and held, and as described above, the fin attachment mechanism can be attained by the present invention.

Alternatively, as in the invention according to claim 2, the fin attachment structure may be one in which the elastic member is a coil spring.

Alternatively, as in the invention according to claim 3, the fin attachment structure may be one in which the ball part is configured by a spherical body.

Alternatively, as in the invention according to claim 4, the fin attachment structure may be one in which the length in the lengthwise direction of the groove part of the surfboard is longer than that in the lengthwise direction of the fin attachment base portion.

Alternatively, the above-described problems can be solved similarly by the use of the fin attachment structure in which, as in the invention according to claim 5, the hollow parts are provided at two portions, that is, in the front direction and in the rear direction of the groove part and at the same time, the hollow parts are provided also at the opposing positions on both side surfaces of the groove part, respectively, and by the insertion of the fin attachment base portion into the groove part, the ball parts in the hollow parts provided on both sides at the two portions in the front direction and in the rear direction of the groove part are moved into the hollow parts against the biasing of the elastic member and thus the fin is biased by the ball part and is sandwiched and held.

Alternatively, as in the invention according to claim 6, the fin attachment structure may be one in which the groove part is provided in the fin box and the fin box is mounted to the bottom surface of the surfboard.

In addition to these, as in the invention according to claim 7, the fin, which is used in the fin attachment structure according to any of claims 1 to 6, having recess portions corresponding to the projected surface of the ball part on both side surfaces, respectively, of the fin located at the ball part when the fin is attached in the groove of the fin attachment base portion may be used to magnify the sandwiching and holding effect.

Alternatively, as in the invention according to claim 8, the fin may be used, in which either or both of the corner portions in the front direction and in the rear direction of the fin attachment base portion are configured by the inclined side part, or the arc side part may be used instead of the inclined side part as in the invention according to claim 9.

From the above configurations, first, the invention according to claims 1 to 3 makes it possible to attach the fin to the surfboard both detachably and easily and at the same time, to attach the fin without using any tools.

Along with this case, because the fin can be sandwiched and held tightly by an elastic force by the elastic member at the deep side and the ball part at the opening side in the hollow part, it is unlikely that the fin comes off unexpectedly during surfing.

Further, when an expected force is applied to the fin itself, for example, when the fin comes into contact with human body, the impact will release the biasing by the ball part and the elastic member in the hollow part provided on both sides of the groove part in the vicinity where the impact is received, and the fin is allowed to escape from the groove part for fin attachment, and thus the impact of the collision of the fin to human body etc. can be released by the dislodgement of the fin.

Consequently, the attachment/detachment of the fin can be facilitated and at the same time, the damage when the fin hits against human body can be minimized and the damage to the fin can also be reduced.

Further, the fin attachment structure can be easily manufactured and can be manufactured at a low cost, having both high versatility and usability.

Furthermore, according to the invention according to claim 4, by lengthening the groove part, the rotation of the fin can be made more efficient and the dislodgement of the fin when receiving an impact can be reserved by the lengthened part.

Next, according to the invention according to claim 5, the fin can be sandwiched and held by the elastic member and the ball part both in the front direction and in the rear direction of the fin, and thus the fin can be fixed more securely and at the same time, can be sandwiched and held more efficiently.

Further, even when the fin receives an impact in either of the front and rear directions, the sandwiched and held part in the vicinity of the impact can be dislodged and thus the impact on the fin can be released and reduced.

Consequently, even when the fin comes into contact with human body etc., the sandwiched and held fin is dislodged, and the impact can be reduced and the damage, such as breakage, can be minimized.

In particular, in the sandwiched and held region other than the region from which the sandwiched and held fin is dislodged, the sandwiched and held fin is still maintained, and therefore, the fin is not lost and can be accommodated in a state of being suspended from the board.

Further, when the sandwiched and held portion that has received an impact is dislodged, the impact is changed into a force to rotate the fin with other sandwiched and held portion as its axial center, that is, the impact is released into the rotation of the fin with the sandwiched and held portion of the fin as its axial center, and thus the force in the direction of the impact can be released extremely efficiently and can be reduced considerably.

Consequently, even when the fin hits against human body, the fin will just rotate and separate from the human body, and thus its impact can be almost eliminated

Next, according to the invention according to claim 6, the fin box can also be used and the attachment to the board can be simplified and at the same time, its manufacture is facilitated.

Further the versatility is increased and at the same time various adjustments are made vary easy.

Next, according to the invention according to claim 7, the ball part can be inserted tightly into the recess portion of the fin and therefore, the sandwiching and holing force can be increased and at the same time, the fin can be installed in place at the predetermined position when mounted.

Alternatively, according to the invention according to claim 8 or 9, the inclined side part or the arc side part can facilitate the rotation of the fin attachment base portion in the groove part and thus it can rotate efficiently when receiving an impact.

In addition, it is made easy to rock the fin back and forth at the time of attachment/detachment and thus its handling can be made easier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a fin attachment structure according to the present invention.

FIG. 2 is a diagram showing an example of a state where a fin is attached to the fin attachment structure according to FIG. 1.

FIG. 3 is a cross-sectional view along A-A line in FIG. 2.

FIG. 4 is a diagram showing an example of a state where the sandwiched and held fin is dislodged and the fin has rotated from the state shown in FIG. 2.

FIG. 5 is a diagram showing an example of a state where the sandwiched and held fin is dislodged and the fin has rotated from the state shown in FIG. 2.

FIG. 6 is a diagram showing another example of a fin attachment structure according to the present invention.

FIG. 7 is a diagram showing another example of a fin attachment structure according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagram showing a basic example of a fin attachment structure according to the present invention, an example of a fin attachment part that is part of the bottom surface of a surfboard.

That is, the fin attachment part of a surfboard has a groove part 1 for fin attachment and a fin 5 is inserted into the groove part 1.

The inner surface of the groove part has a ball part 3 biased by a coil spring 2 so as to sandwich both sides of the fin 5 inserted into the groove part within the groove part 1.

The coil spring 2 and the ball part 3 for sandwiching and holding a fin are mounted within a hollow part 10 provided in the inner surface of the groove as shown in the figure, which has the coil spring 2 at the deep side and the ball part 3 at the opening side.

For example, a plunger may, of course, be used.

This further has a pair respectively on both sides in the lengthwise direction of the groove part 1 and the fin 5 inserted into the groove part is sandwiched and held from the direction of both sides at two portions, that is, at two portions in the front direction and in the rear direction, respectively.

Further, the length in the lengthwise direction of the groove part 1 of the surfboard is longer than that in the lengthwise direction of an attachment base portion 51 of the fin 5 and it is necessary to have a space both in the front direction and in the rear direction of the groove part when the fin 5 is attached.

In the case where the attachment base portion 51 of the fin 5 is inserted into the groove part 1 for fin on the bottom surface of the surfboard with the configuration described above, the attachment base portion 51 of the fin 5 is sandwiched and held in the direction of both sides at two portions in the front direction and in the rear direction with respect to the board, as a result.

In this case, the space described above allows the insertion of the fin 5 in the state where the fin 5 is caused to fall down in the forward direction or in the backward direction and at the same time, the detachment thereof by causing the fin 5 to fall down in the forward direction or in the backward direction at the time of detachment.

In this case, the ball part 3 biased by the coil spring 2 sandwiches and holds the fin 5 and due to this, the fin 5 is fixed tightly.

The attachment base portion 51 of the fin 5 at which the ball part 3 is biased when the fin 5 is attached has a spherical recess portion 52 corresponding to the spherical surface of the ball part 3.

Due to this, the fin can be fitted into tightly and fastened to the ball part 3 of the surfboard.

In particular, the fin is pressed from both sides and at the same time, is sandwiched at two portions at the front and rear, and therefore, the fin can be fixed tightly and is unlikely to come off during surfing etc.

At the time of attachment/detachment, by holding and pressing in an arbitrary portion of the front end of the fin 5, the ball part 3 at the front of the coil spring 2 is pressed into the hollow part 10 provided in both inner sides of the groove part 1 of the surfboard and the attachment base portion 51 of the fin 5 can be inserted into the groove part 1.

Further, the ball part 3 is fitted into to be attached to the recess portion 52 of the fin 5 and thereby the fin is positioned securely and fixed at a predetermined position.

In this case, the ball part 3 is biased by the coil spring 2 and at the same time, the ball part 3 is fitted into to be attached to the recess portion 52 that matches with the shape of the ball part 3, and therefore, the fin is fixed in place tightly and unlikely to come off unexpectedly during surfing.

On the other hand, when a force is applied to the fin 5 in the forward direction or in the backward direction, that is, when the surfboard moves in the direction in which the fin 5 moves or in the direction opposite to the direction in which the fin 5 moves, that is, in the nose direction of the board, and if the fin 5 hits against something, the ball part 3 biased by the coil spring 2 in the direction in which the force is applied comes off the recess portion 52 of the fin as a result.

In this case, the recess portion 52 of the fin sandwiched and held by the ball part 3 biased by the other coil spring 2 is maintained and the fin 5 rotates with the sandwiched and held portion of the recess portion 52 that is still maintained as its axial center.

Consequently, for example, even when the fin 5 hits against human body, the fin 5 comes off the attachment part and the impact on the human body can be avoided.

In particular, the fin 5 is sandwiched and held by the coil spring 2 that biases the fin 5, and when a large force is applied unexpectedly and the fin 5 is broken or it hits against human body, the impact can be released by the partial dislodgment of the fin 5 from the board without the damage to the human body. Further, part of the fin 5 remains still sandwiched and held by a pair of the bail parts 3 in the other direction and therefore the loss of the fin can be prevented.

FIG. 2 is a diagram showing an example in which the fin 5 is attached on the board that has the fin attachment mechanism according to the present invention, also showing an example of a side view of a state where the fin 5 is attached on the bottom surface of the board.

As shown in the figure, the fin 5 is sandwiched and held and thus fastened by the ball part 3 biased by the spring 2 at two portions in the front direction and in the rear direction, respectively, of the attachment base portion 51 of the fin 5.

In this case, both corner portions to the front direction and in the rear direction, respectively, of the fin attachment base portion 51 are cut off and configured by an inclined side part 53.

In this manner, by configuring both end portions of the attachment base portion 51 by the inclined side not by the corner portion, it is possible to dislodge the fin sandwiched and held by the ball part 3 fitted into to be attached thereto by causing the fin to fall down forward or backward.

In the figure, the end portion is configured by the inclined side part 53, however, it may be configured by an arc side part instead of this.

The ball part 3 and the coil spring 2 are mounted within the hollow part 10 provided on both side parts of the groove part 1 of the surfboard, however, a configuration in which the groove part 1 is formed directly on the main body of the board and the hollow part 10 is further provided in the groove part 1, or a configuration in which a fin box having the groove part 1 and the hollow part is configured in advance and then this is mounted on the board may be accepted.

Although an example is shown, in which the length in the lengthwise direction of the groove part 1 of the surfboard is configured so as to be longer than that in the lengthwise direction of the fin attachment base portion 51 and when the fin 5 is attached, a space is provided in the front direction and in the rear direction, respectively, of the groove part 1, the interval of the space is not limited to this and a longer space may be provided.

FIG. 3 shows a cross-sectional view along A-A line in FIG. 2.

As shown in the figure, the hollow parts 10 are provided respectively at the opposing positions on both sides of the inner surface of the groove part 1 of the board, the coil spring 2 is provided in the hollow part 10, and the ball part 3 is provided in the direction of the opening of the hollow part 10.

Due to this, both sides of the fin attachment base portion 51 inserted into the groove part 1 are sandwiched and held.

In this case, the recess portion 52 is provided at the corresponding portion of the fin 5 and thus the ball part 3 is tightly fitted into to be attached.

The shape of the hollow part 10 shows only an example and the shape is not limited to this.

The coil spring 2 is explicitly shown, however, this is not limited and, for example, the coil spring 2 may be one, such as a resin material or rubber that has elasticity, or an elastic member that can bias in the direction toward the opening, such as a plate spring, or a spring member in another form, and which has an elastic compressing force capable of at least fixing the fin 5.

Consequently, the coil spring 2 may be one that uses either of elastic members capable of biasing, such as an elastic member having a structure that gives oil pressure, hydraulic pressure, or air pressure, or another elastic member capable of biasing in a variety of ways.

The ball part is only required that at least its contact surface with the fin 5 be formed into the shape of a projection portion and its shape is not limited to a spherical body.

Consequently, the shape having at least a projection portion is represented as the ball part 3 in the present specification and used in this meaning.

FIG. 4 explicitly shows an example of the fin 5 by the solid line in the state where a force due to contact etc. is applied from the rear of the fin 5 and the fin 5 sandwiched and held in the rear direction comes off the surfboard from the state where the fin 5 expressed by the alternate long and two short dashes line is mounted on the surfboard.

As shown in the figure, by the dislodgment of the sandwiched and held fin 5, the impact by the fin 5 can be reduced and, for example, when it comes into contact with human body, it is possible to avoid the damage to the human body or restrict the damage to a minimum.

Further, the impact can be avoided in this manner, and therefore, the damage to the fin itself can also be restricted to a minimum.

In particular, the corner portion 52 in the front direction of the fin attachment base portion 51 is configured by an inclined side part 53, and therefore, the rotation of the attachment base portion 51 of the fin is facilitated within the groove part 1.

Further, only part of the sandwiched and held fin 5 is dialodged, and other part is still sandwiched and held, and therefore, it is possible to prevent the loss of the fin 5.

FIG. 5 explicitly shows an example of the fin by the solid line in the state where a force due to contact etc. is applied from the front of the fin 5 and the fin 5 sandwiched and held in the front direction comes off the surfboard from the state where the fin expressed by the alternate long and two short dashes line is mounted on the surfboard.

In this figure also, the corner portion in the rear direction of the fin attachment base portion 51 is configured by the inclined side part 53, and therefore, when the sandwiched and held part in the front direction is dislodged, the rotation of the fin 5 in the backward direction is facilitated.

In this specification, a position is specified using the term “front” and “rear”, however, when a position is specified on the explicit basis, it should be obeyed and when a position is not specified in particular, the nose direction of the surfboard, that is, the direction in which the surfboard is moving is referred to as the front direction and the tail direction is referred to as the rear direction.

FIG. 6 shows an example in which the fin box has the fin attachment structure according to the present invention and the fin box has the groove part 1 into which the fin attachment base portion 51 is inserted.

The groove part 1 has a certain space at the front and the rear of the fin 5, respectively, and at the same time, the hollow parts 10 at the opposing positions in the front ansa rear directions of the inner surface, respectively.

In this case, the hollow part 10 has the coil spring 2 and the ball part 3 therein.

Further, the hollow part 10 may have therein a spring adjustment mechanism, that is, a mechanism to change the depth of the hollow or the compression interval of the coil spring, allowing to change the strength of the spring.

By mounting the fin box on the surfboard, the fin attachment mechanism according to the present invention can be used.

The hollow part 10 shown in this figure has a configuration in which the bottom part is removed and a hexagonal groove for adjusting the spring or coil spring pressure in the direction of the bottom part is provided, however, even if the bottom part is removed from the fin box is also expressed as the hollow part 10 and specified in this specification.

FIG. 7 is a diagram showing an example in which the fin box similarly has the fin attachment structure according to the present invention and the fin box has the groove part 1 having an arbitrary length at least double or longer than the length of the fin attachment base portion 51, and the hollow parts 10 are provided at equal intervals at three portions from the front direction to the rear direction on the inner surface of the groove part 1 and the hollow part 10 has the coil spring 2 and the ball part 3 in the figure.

The hollow part 10, the coil spring 2, and the ball part 3 are similarly provided on the inner surface of the groove part 1 on the opposite surface opposing the hollow part 10 on one side surface of the groove part 1 and thereby the fin attachment base portion 51 can be sandwiched and held by being sandwiched from both sides.

In this figure, the fin 5 is sandwiched and held by the coil spring 2 and the ball part 3 opposing the hollow parts 10 at two portions at the intermediate portion and the rear portion of the groove part 1 and nothing is sandwiched and held by the coil spring 2 and the ball part 3 opposing the hollow part 20 in the front direction.

With such a configuration, the fin can be attached to any position.

-   -   1 groove part     -   10 hollow part     -   2 coil spring     -   3 ball part     -   5 fin     -   51 fin attachment base portion     -   52 recess portion     -   53 inclined side part 

1. A fin attachment structure, which is a fin attachment mechanism for attaching a fin detachably to a bottom surface of a surfboard, comprising: a groove part 1 into which a fin attachment base portion 51 is inserted on the bottom surface of the surfboard and a hollow part 10 on the inner surface of the groove part 1, the hollow parts 10 being provided at the opposing positions on both side surfaces of the groove part 1, respectively; and an elastic member 2 at the deep side within the hollow part 10 and a ball part 3 at the opening side, the ball part 3 being biased by the elastic member 2 from the opening within the hollow part 10 in the direction in which it projects to outside, wherein by the insertion of the fin attachment base portion 51 into the groove part 1, the ball parts 3 within the hollow parts 10 provided on both sides of the groove part 1 is moved into the hollow part 10 against the biasing of the elastic member 2 and thereby the fin 5 is biased and sandwiched and held by the ball part
 3. 2. The fin attachment structure according to claim 1, wherein the elastic member 2 is a coil spring.
 3. The fin attachment structure according to claim 1, wherein the ball part 3 is configured by a spherical body.
 4. The fin attachment structure according to claim 1, wherein the length in the lengthwise direction of the groove part 1 of the surfboard is longer than that in the lengthwise direction of the fin attachment base portion
 51. 5. The fin attachment structure according to claim 1, wherein the hollow parts 10 are provided at two portions in the front direction and in the rear direction of the groove part 1 and at the same time, also provided at opposing positions on both side surfaces of the groove part 1, respectively; and by the insertion of the fin attachment base portion 51 into the groove part 1, the ball parts 3 within the hollow parts 10 provided on both sides at two portions in the front direction and in the rear direction of the groove part 1, respectively, are moved into the hollow part 10 against the biasing of the elastic member 2 and thereby the fin 5 is biased and sandwiched and held by the ball parts
 3. 6. The fin attachment structure according to claim 1, wherein the groove part 1 is provided in a fin box and the fin box is mounted on the bottom surface of the surfboard.
 7. A fin used in a fin attachment structure according to claim 1, wherein a fin attachment base portion 51 has a recess portion 52 corresponding to a projected surface of the ball part 3 respectively on both side surfaces of the fin 5 located at the ball part 3 when the fin 5 is mounted in the groove part
 1. 8. A fin used in the fin attachment structure according to claim 1, wherein either or both of corner portions in the front direction and in the rear direction of the fin attachment base portion 51 are configured by an inclined side part
 53. 9. The fin according to claim 8, wherein instead of the inclined side part 53, an arc side part is used. 